First published in 2001. This is Volume 7 in a series of ten on the Science of Mental Health. One of the most challenging areas of behavioral research is the study of personality and personality disorders. The main challenge can be stated directly: it is difficult to know with certainty which personality traits are fundamental and which are complex elaborations of fundamental traits. This is a collection of works under the sections of Description, Epidemiology, Genes and Environment, Peers and Neighborhoods, Neurobiology and Behavior and Treatment.